Chromatography is the printing in the case that the corresponding region of the previous color plate is knocked out during the printing process. Misregistration often occurs in multicolor chromatography, especially in flexography. Misregistration causes gaps to appear in the edge of two adjacent color blocks. In order to avoid those gaps, it is necessary to generate some small overlaps, also known as trapping regions, between the edges of different objects so that they are unnoticeable to human eyes.
Trapping is customarily called amending color trapping in the printing industry. It is mainly to amend the boundaries between two adjacent different colors caused by misregistration. People always feel that dark color is closer to eyes than light color when looking at a printing. Therefore, when trapping is made to original printings, people always try to make the light color to be covered by the dark color, and keep the dark color the same. Thus, even if a small offset appears due to misregistration, the overlapped printing would prevent the shape of the objects from being distorted.
There are two major trapping methods. One is manual trapping. Manual trapping is neither efficient nor effective. Another method is automatic trapping based on adjacent colors only. That trapping method takes account of adjacent color block pairs only. Although it can make one-to-many trapping regions connections when trapping borders are adjacent, and trapping direction are the same, it can not make many-to-one or many-to-many trapping regions connections. U.S. Pat. No. 7,123,381B2 discloses a trapping method based on a single trapping border, but it fails to make a connection of all trapping borders. Thus, the automatic trapping method based on adjacent color block pairs needs to be improved.